Exhaust gas recirculation represents an important manner for lowering the nitrogen oxide emissions which are generated during the combustion of fuel in internal combustion engines. Here, a portion of the exhaust gas is recirculated into the intake chamber of the internal combustion engine. Recirculating a portion of the exhaust gas which, as an inert gas, is sluggish in reaction, reduces the generation of nitrogen oxides by lowering the peak temperatures during combustion. Normally, the exhaust gas is recirculated into the intake chamber by mixing a portion of the exhaust gas with the fresh intake air. An exhaust gas recirculation valve is provided for metering the exhaust gas. In addition to a high pressure exhaust gas recirculation system situated close to the engine, a low pressure exhaust gas recirculation system is also known in which, generally, the exhaust gas recirculation duct is implemented downstream from a diesel particle filter and upstream from a compressor.
In order to control the exhaust gas recirculation system it is necessary to record the various mass flows.
Normally, the mass flow via the low pressure exhaust gas recirculation system is modeled, the low pressure exhaust gas recirculation mass flow being determined with the aid of a so-called throttle equation, into which the pressure upstream and the pressure downstream from a low pressure exhaust gas recirculation valve, the temperature upstream from the low pressure exhaust gas recirculation valve and the geometry, i.e., the cross section of the low pressure exhaust gas recirculation valve are incorporated. The cross section of the low pressure exhaust gas recirculation valve is normally not determined directly, but is fixed, for example, with reference to the known position of the valve. An exemplary embodiment for the calculation of the mass flow with the aid of the throttle equation is described, for example in the unexamined patent application DE 10 2008 040 633 A1.
Patent application DE 10 2010 027 976 A1 describes a calculation of a low pressure mass flow, on the basis of which the mass flow flowing through the compressor and the fresh air mass flow fed to the internal combustion engine are ascertained. The low pressure mass flow is obtained by a difference formation. In this case, the low pressure mass flow flowing through the compressor is ascertained based on a model which takes into account the speed of the compressor and/or a pressure upstream from the compressor and/or a pressure downstream from the compressor and/or a temperature upstream from the compressor and/or a temperature downstream from the compressor. Corresponding calculations of the mass flow via the compressor are also described, for example, in unpublished patent application DE 198 04 330 A1 and unpublished patent application DE 101 58 250 A1.
The calculations require the fresh air mass flow fed to the internal combustion engine, which is normally measured using an air mass flow sensor situated in the intake area.